This invention generally relates to tundish slide gate valves used to regulate a flow of molten steel, and is specifically concerned with a unitized porous nozzle and top plate for use in such a valve.
Porous nozzles for use in tundish slide gate valves are well known in the prior art. The walls of such nozzles are formed from a porous, gas permeable refractory material which may be a ceramic oxide of aluminum, silicon, magnesium, chromium, or zirconium, or mixtures thereof. The inside surface of the nozzle walls defines a bore for conducting a flow of liquid metal such as steel. The outside surface of these nozzles is enveloped in a "can" of metallic sheet material, such as steel, that is spaced apart from the outside nozzle surface in order to define one or more annular, gas conducting spaces.
Such prior art nozzles are installed within the nozzle well of a tundish. A metal jig is used to position the discharge end of the nozzle in alignment over a circular opening in a top plate. A high temperature gasket is placed between the bottom flat surface of the discharge end of the nozzle and a circular area around the top plate opening prior to the positioning of the top plate under the nozzle to join and seal the nozzle bore in registry with the top plate opening. The top plate overlies a slidable throttle plate having a circular opening that is registrable with the circular opening in the top plate to modulate the flow of steel in gate valve fashion.
In operation, pressurized inert gas is permeated through the annular space between the outside surface of the nozzle and the steel can that circumscribes it while molten metal flows through the bore of the nozzle. The inert gas flows through the porous nozzle walls, and advantageously forms a fluid film over the surface of the bore within the nozzle that prevents that molten metal from making direct contact with the inner surface forming the bore. By insulating the bore surface from the molten metal, the fluid film of gas prevents the small amounts of alumina that are present in such steel from sticking to and accumulating onto the surface of the nozzle bore. The prevention of such alumina deposits is important, as such deposits will ultimately obstruct the flow of molten steel until it congeals around the walls of the bore, thereby clogging the nozzle. Such a clogged nozzle necessitates the shutting down of the slide gate valve and the replacement of the nozzle.
While such porous nozzles have generally shown themselves to be effective in retarding the accumulation of bore-obstructing alumina deposits, the inventors have observed a number of shortcomings associated with such nozzles which have prevented them from realizing their full potential. For example, while the inert gas permeates through the porous walls of such nozzles is effective in retarding alumina deposits on the surface of the nozzle bore itself, none of the argon gas forms any kind of effective, deposit-retarding film on the surface of the opening in the top plate of the valve. Consequently, alumina deposits are apt to form around the surface of the circular discharge opening in the top plate, which can ultimately lead to valve obstruction. Still another shortcoming follows from the mounting of the inert gas coupling assembly directly in the steel can that circumscribes the outer surface of nozzles. The female portion of the coupling assembly that is typically welded on the steel can of the nozzle can attain temperatures of up to 1800.degree. F., causing it to expand relative to the male portion of the coupling to an extent where a major gas leak can result. Such a leak can jeopardize the function of the gas in penetrating the nozzle walls and forming a protective fluid film over the surface of the nozzle bore, as the pressure of the inert gas must be maintained at a level high enough to overcome the considerable back-pressure that the molten steel applies to the surface of the bore. Ideally, the gas pressure should be just enough to form the desired film. If it is too high, the gas can stir the steel excessively, thus creating additional defects. Thus the control of the gas pressure and flow is critical, and must be maintained within a narrow range. Any significant leak can jeopardize the desired delicate pressure balance. Other shortcomings of prior art nozzles occur as a result from the in situ formation of the butt joint between the fiat bottom surface of the discharge end of the nozzle, and the area surrounding the flow opening in the top plate. The metal jigs used to position and align the nozzle over the top plate can warp, leading to misalignments. Moreover, the fact that the positioning of the nozzle over the top plate is performed in situ between the tundish and the throttle plate, generally makes the installation process clumsy, inconvenient, and time consuming. Finally, the resulting misalignments that can occur between the nozzle and the top plate can result in uneven pressure on the material forming the gasket between these components. Such uneven pressure can result in large variations in gasket thickness, which in turn can lead to a gasket failure and a consequent break-out of steel, or unwanted raising of the nozzle.
Clearly, there is a need for an improved kind of nozzle mechanism that prevents or at least minimizes the accumulation of alumina deposits on the nozzle bore, and prevents gas leaks from occurring in the gas coupling leading to the nozzle can. Ideally, such an improved nozzle mechanism could be easily and quickly installed in a tundish slide gate valve without the need for jigs or other alignment mechanisms, and further without the need for the formation of an in situ gasket. Finally, it would be desirable if such an improved nozzle assembly were reliable, durable, and relatively inexpensive to manufacture.